When packet data is transmitted, several packet data are gathered to form a data having a fixed length, and sync patterns are added to the data before transmission. A receiver detects the sync patterns and then extracts the packet data. This is a known method in this field. In other words, this data stream comprises a plurality of data placed between the sync patterns disposed at fixed intervals.
In general, the receiver detects the sync patterns, and when two sync patterns at a given intervals are detected, these two patterns are determined right sync patterns, then the receiver starts securing the sync referencing the positions of the sync patterns.
A flywheel fashion is one of the securing methods, and according to the flywheel fashion, when right sync patterns are detected, windows are opened at the given intervals referencing the detected positions, then only the sync patterns are examined through the windows again. This is a known method in the field, and is used not only in communication but also in VCRs for sync security. However, this method may encounter the following problems.
A series of sync patterns recognized as right sync patterns ignites the flywheel action for sync security. If sync patterns are detected in series at wrong places, sync security is activated on a phase independent of data transmission, and as a result, wrong data is received.
One of the major factors in detecting wrong sync patterns is the problem of generating so called ‘pseudo sync pattern’. In other words, the same pattern as the sync pattern occur with a certain probability in the data being transmitted. When a signal, of which bit-structure cannot be predicted, such as a video signal or an audio signal is transmitted, this problem is inevitable. In particular, when a fixed pattern occurs at a fixed position during the data transmission, the pseudo sync patterns thus occur at fixed intervals. Therefore, the pseudo sync pattern may start the flywheel action. As a result, the receiver receives totally different data.
In order to solve this problem, a plurality of different sync patterns are used. Japanese Patent Application Non-examined Publication No. S61-168173 discloses the method of using plural types of sync patterns. Although using plural different sync patterns, this prior art places plural different sync patterns in series with respect to sequential data thereby recording or playing back the data, i.e. prolong the sync pattern thereby reducing the probability of the pseudo sync pattern. Therefore, as long as the pattern occur at fixed intervals during the data transmission, this prior art cannot solve the problem, but adversely it increases data redundancy, which entails a larger size of circuit.
In other words, to realize a communication method eliminating the pseudo sync pattern and allowing reliable sync security, conventional methods have inherent problems discussed above, i.e. prolonging a bit length of the sync pattern thereby increasing the data redundancy, which needs a larger circuit.